In order for an imaging print support to be widely accepted by the consumer for imaging applications, it has to meet requirements for preferred basis weight, caliper, stiffness, smoothness, gloss, whiteness, and opacity. Supports with properties outside the typical range for ‘imaging media’ suffer low consumer acceptance.
In addition to these fundamental requirements, imaging supports are also subject to other specific requirements depending upon the mode of image formation onto the support. For example, in the formation of photographic paper, it is important that the photographic paper be resistant to penetration by liquid processing chemicals. Lack of adequate resistance to liquid penetration will result in poor image quality. In the formation of ‘photo-quality’ inkjet paper, it is important that the paper is readily wetted by ink and that it exhibits the ability to absorb high concentrations of ink and dry quickly. If the ink is not absorbed quickly, the print will stick together when stacked against subsequent prints and exhibit smudging and uneven print density. It is also important that the print remain dimensionally stable in all dimensions including the thickness or z directional plane. Papers that become nonplanar, or wavy like a potato chip, upon absorption of ink are said to have “cockled”. For thermal media, it is important that the support contain a thermally insulative layer in order to maximize the transfer of dye from the donor, which results in a higher color saturation.
It is important, therefore, for an imaging media to simultaneously satisfy several requirements. One commonly used technique in the art for simultaneously satisfying multiple requirements is through the use of composite structures comprising multiple layers wherein each of the layers, either individually or synergistically, serves distinct functions. For example, it is known that a conventional photographic paper comprises a cellulose paper base that has applied thereto a layer of polyolefin resin, typically polyethylene, on each side, which serves to provide waterproofing to the paper and also provides a smooth surface on which the photosensitive layers are formed. In another imaging material as in U.S. Pat. No. 5,866,282, biaxially oriented polyolefin sheets are extrusion laminated to cellulose paper to create a support for silver halide imaging layers. The biaxially oriented sheets described therein have a microvoided layer in combination with coextruded layers that contain white pigments such as TiO2 above and below the microvoided layer. The composite imaging support structure described has been found to be more durable, sharper, and brighter than prior art photographic paper imaging supports that use cast melt extruded polyethylene layers coated on cellulose paper. In U.S. Pat. No. 5,851,651, porous coatings comprising inorganic pigments and anionic, organic binders are blade coated to cellulose paper to create ‘photo-quality’ inkjet paper.
In all of the above imaging supports, multiple operations are required to manufacture and assemble all of the individual layers. For example, photographic paper typically requires a papermaking operation followed by a polyethylene extrusion coating operation, or as disclosed in U.S. Pat. No. 5,866,282, a paper-making operation is followed by a lamination operation for which the laminates are made in yet another extrusion casting operation. There is a need for imaging supports that can be manufactured in a single in-line manufacturing process while still meeting the stringent features and quality requirements of imaging bases.
It is also well known in the art that traditional imaging bases consist of raw paper base. For example, in typical photographic, approximately 75% of the weight of the photographic paper comprises the raw paper base. Although raw paper base is typically a high modulus, low cost material, there exist significant environmental issues with the paper manufacturing process. There is a need for alternate raw materials and manufacturing processes that are more environmentally friendly. Additionally to minimize environmental impact, it is important to reduce the raw paper base content, where possible, without sacrificing the imaging base features that are valued by the customer, i.e., strength, stiffness, surface properties and the like, of the imaging support.
An important corollary of the above is the ability to recycle photographic paper. Current photographic papers cannot be recycled because they are composites of polyethylene and raw paper base and, as such, cannot be recycled using polymer recovery processes or paper recovery processes. A photographic paper that comprises significantly higher contents of polymer lends itself to recycling using polymer recovery processes.
Existing composite color paper structures are typically subject to curl through the manufacturing, finishing, and processing operations. This curl is primarily due to internal stresses that are built into the various layers of the composite structure during manufacturing and drying operations, as well as during storage operations (core-set curl). Additionally, since the different layers of the composite structure exhibit different susceptibility to humidity, the curl of the imaging base changes as a function of the humidity of its immediate environment. There is a need for an imaging support that minimizes curl sensitivity as a function of humidity, or ideally, does not exhibit curl sensitivity.
The stringent and varied requirements of imaging media, therefore, demand a constant evolution of material and processing technology. The idea of an all polymer “synthetic” paper has been around for years. Use of these synthetic papers has been tried in imaging media with limited success (U.S. Pat. No. 5,275,854.) However, these “papers” traditionally fail to meet imaging needs for a variety of reasons such as weak, plastic-like feel and relatively high cost. Synthetic papers in the past have been 3 to 4 times more expensive than media manufactured from cellulose fiber. Schut, J. H., “The New Look in Plastic—It's Paper!”, Plastics Technology, Gardner Publications, Inc., New York, N.Y., February 2000. As the technology has improved, costs have dropped such that these papers are now within the current range of fine printing papers. There are still barriers to entry into the imaging arena, however, pertaining to stiffness, opacity, conductivity, and surface roughness. Stiffness tends to be the primary feature where synthetic papers compare poorly to cellulose containing media. P-172 Synthetic Paper Industry Report, pg. 3, Business Communications Co., Inc., Norwalk Conn., March 2001. In order to meet cost requirements, the newly introduced synthetic papers (e.g. Japanese Patent 2000211008) typically are comprised of polyethylenes and polypropylenes. Due to the inherently lower elastic modulus of these materials, the stiffness for a sheet of any given comparable weight is at a significant disadvantage compared to a paper base of approximately the same weight. It is well known that stiffness of an imaging element is a function of the modulus of the various layers of the imaging element, the location of the various layers (particularly in terms of the distance from the bending axis) and the overall caliper of the imaging element. Improvements that can be made to the modulus of the various layers comprising the imaging element can increase the overall bending stiffness of the element thus, in turn, increasing its value as an imaging support.
U.S. patent application Ser. No. 09/723,518 presents a foam core imaging member that has adhered to each side a flange sheet comprising extruded or stretched polyolefin. Although this element exhibits the required stiffness associated with that of an imaging element, there is a problem with this in that the surface roughness of the element, which is a function of the surface roughness of the foam core, is poor. The poor surface roughness is inherent to the foaming process wherein the rate of quenching, chill roller surface, blowing agent concentration, additional additives, and polymer matrix material all play a significant role in foam surface quality. Accordingly, it has limited application as an imaging base.
Organic additives that have the potential to enhance the modulus of oriented polyolefin film are known in the art. The composition of the organic additive, which is typically a hydrocarbon resin, must be such that it exhibits a higher glass transition temperature (Tg) than polypropylene. It must also be compatible with polypropylene. It is believed that the addition of the organic additive increases the Tg of the amorphous polypropylene, leading to a densification of the amorphous phase over time, which leads to increased stress transfer between crystalline regions (also called a pseudo network effect) that, in turn, leads to increasing stiffness. For example, Bossaert et al. in U.S. Pat. No. 4,921,749 claim a polyolefin film comprising a base layer of 70% to 97% polypropylene and 30% to 3% hydrogenated resin. The addition of about 20% hydrogenated resin is shown to result in an increase in modulus of about 10–20%. Klosiewicz, in U.S. Pat. No. 6,281,290 claims a process for producing a master batch for a polypropylene article (film, fiber, sheet, or bottle) comprising a mixture of polypropylene, high density polyethylene and hydrocarbon resin having a ring and ball softening point of at least 70 degrees Centigrade. The addition of low levels of hydrocarbon resin and high density polyethylene (HDPE) are reported to increase the tensile modulus of extrusion cast oriented polypropylene films by 15% to 70%.
Traditional imaging elements derive a predominant fraction of their bending stiffness from the cellulose paper substrate and as such do not require the use of organic stiffening additives. However, in the case of non-cellulose core imaging elements, there is potentially a significant application of such technology if it is shown to be viable for polyolefin elements and for extrusion coating processes. C-S Liu, in U.S. Pat. No. 4,365,044, discloses an extrusion-coatable polypropylene composition comprising a hydrogenated copolymer of vinyl toluene, alpha-methyl styrene, and low density polyethylene. Extrusion coatability at speeds up to about 275 m/minute and good adhesion to cellulose substrates, is claimed. However, such a composition is not suitable for use in an imaging element.
Opacity can also be a limiting factor for many of the available all synthetic materials. Usually these materials can not provide a comparable opacity to cellulose bases unless excessive levels of fillers are used. In some cases up to 40% CaCO3 is used, as described in JP 2000211008, which would undoubtedly cause contamination problems in low pH photofinishing solutions used for photographic applications. Use of an all synthetic base, particularly in silver halide applications, would also require a new method of providing adequate conductivity control throughout the manufacturing process. Since an all polymer support does not conduct electrical charges as well as a cellulose paper containing 4–10% moisture, a new method of static protection would be needed to avoid abrupt static discharges. There are several static problems, such as linting, which occur with synthetic papers in the printing industry. Ducey, Michael J., “Synthetic Paper is Coming of Age”, Graphics Arts Monthly, December 1999, Cahners Publishing Co. Uncontrolled static buildup and release is a distinct disadvantage for synthetic “papers”, particularly if used in photographic media, which comprise a light sensitive emulsion. Fast finishing speeds, up to 600 m/min. make this a serious converting concern as well. Static attraction problems may also result in multiple sheet feed problems.
Coating on polymer films rather than cellulose paper has been known to improve several surface characteristics, such as “orange peel”. “Orange peel” arises primarily from the surface non-uniformity of the paper formation, this non-uniformity becomes more noticeable and therefore more objectionable the glossier the surface. As the resin coating layers become thinner, “orange peel” and the natural roughness of the cellulose paper fibers are more apt to become objectionable. While polymer films (including some synthetic papers with low Ra values) would offer advantages for an imaging media on the image side, the lack of roughness on the backside of these papers can cause tremendous transport problems throughout the manufacturing process. The surface roughness measurement is a measure of the maximum allowable roughness expressed in units of micrometers and by use of the symbol Ra. For the irregular profile of the backside of photographic materials of this invention, the roughness average, Ra, is the sum of the absolute value of the difference of each discrete data point from the average of all the data divided by the total number of points sampled. Too low of a surface roughness Ra can result in telescoped rolls, poor wound roll condition, and poor conveyance through manufacturing and printing equipment (including photofinishing equipment).
Additional process barriers to commercially available synthetic “papers” include the chemical interaction of some of the pigmented top layers to photographic development chemistries, and to aqueous solutions found in inkjet applications.
In a non-imaging application in U.S. Patent Application 2002/0015834, H. Biddiscombe also discusses the use of biaxially oriented polymeric films having a core layer comprising a voided homopolymer with a density of not more than 0.70 g/cm3, and at least one substantially non-voided layer on each surface of the core layer. The disadvantage of this structure for an imaging element is that all layers are stretched biaxially therefore limiting composition and functionality of those layers. This is the same disadvantage apparent for U.S. Pat. No. 6,153,367 (Gula, T., et. al), where there is discussion of an integral biaxially oriented polyolefin polymer sheet with a lower layer having a matte surface. In this patent, “any suitable biaxially oriented polyolefin sheet may be used for the base of the invention. Microvoided biaxially oriented sheets are preferred and are conveniently manufactured by coextrusion of the core and surface layers, followed by biaxial orientation, whereby voids are formed around void-initiating material contained in the core layer.” As indicated, this sheet is also coextruded and all layers are stretched simultaneously, limiting composition and functionality of those layers.